Does Water Condense in Hydrophobic Cavities? A Molecular Simulation Study of Hydration in Heterogeneous Nanopores

Abstract

We report a molecular simulation investigation of the hydration thermodynamic behavior of silicalite-1 zeolite, in which an increasing amount of attractive surface defects was introduced. An internal surface defect can either be “weak” or “strong”, depending on the balance between the defect−water and water−water interactions. We found that the hydration behaviors in the presence of weak or strong defects were markedly different. The water hydration process in the presence of weak surface defects is basically a homogeneous nucleation process. The effect of the weak defects is to make the framework more attractive overall, and this leads to the progressive shift of the condensation transition, while the framework remains “hydrophobic” overall. In the case of strong defects, water condensation is characterized by a combination of strong adsorption of a limited amount of water at very low pressure, followed by a plateau in the intermediate pressure range and by a sudden filling of the nanoporous volume at higher pressure. A progressive change from “hydrophobic” to “hydrophilic” behavior is observed as the defect content is increased. This work provides some insights from adsorption thermodynamics into the issue of hydration of geometrically and chemically heterogeneous pore surfaces. We find that a small increase in “hydrophilicity” of the porous framework may turn the pore from being dry to being completely filled with water at saturation conditions.